1. Field of the Invention
The present invention relates to a method of estimating the deteriorations of a reproducing device of a thin-film magnetic head wherein heat shocks are applied to that reproducing device using a laser to give rise to a phenomenon imitating the so-called “thermal asperity” defect.
2. Explanation of the Prior Art
With recent improvements in the plane recording density of magnetic disk systems, there has been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often referred to as the MR (magneto-resistive) device for short) and a recording head having a write-only induction type magnetic device are stacked on a substrate.
For the MR device, there is the mention of an AMR device harnessing an anisotropic magneto-resistive effect, a GMR device tapping a giant magneto-resistive effect, a TMR device making use of a tunnel-type magneto-resistive effect, and so on.
With regard to thin-film magnetic heads having various such magneto-resistive effect devices mounted on them, it has been reported that there is often the thermal asperity defect caused as an inherent one (see JP(A)2005-108306).
The thermal asperity is a phenomenon that occurs when a thin-film magnetic head passes over a magnetic disk plane that is a recording medium while levitating and flying over minute bumps or dents, because the magneto-resistive effect device is heated or cooled via the adiabatic compression and/or adiabatic expansion of air.
Of course, that phenomenon occurs not only in a non-contact state but also in a contact state where the magneto-resistive effect device is in contact with minute bumps or dents on the magnetic disk plane. When the head is in collision with minute asperities, there is a local, vigorous heating occurring due to mechanical vibrations and, at the same time, instantaneous friction. Such local heating is supposed to occur for a very short period in which the head passes over the asperities, and propagate right away to the whole device. When heat shocks propagate as if they were waves, the device is supposed to undergo repeated local expansion and local shrinkage.
How a typical deterioration by the thermal asperity occurs is now explained with reference to FIGS. 7A and 7B with an applied magnetic field as abscissa and the ensuing device resistance change as ordinate. Typical initial characteristics are shown in FIG. 7A. In the initial state with none of the deteriorations of the device, there is a liner resistance change vs. the applied magnetic field.
By contrast, FIG. 7B shows the characteristics of a device deteriorated by the thermal asperity while an HDD (hard disk drive) is practically on the run. In FIG. 7B, there is a stepwise resistance change (kink) vs. the applied magnetic field. From the experience so far, it has been found that most of the deteriorations caused by the thermal asperity occur in this mode.
In a possible deterioration model, it would appear that a part of hard bias layer is flipped over by heat shocks. A magneto-resistive effect film does not show any linearity in general, and a given magnetic field is applied to it from an externally located hard bias layer to keep the linearity of its characteristics. With a part of the bias layer flipped over, however, the bias magnetic field wanes resulting in the inability to give good enough bias magnetic field to the magneto-resistive effect device: this could render the characteristics nonlinear.
The situations being like this, it is required that in products fabrication processes, devices likely to undergo characteristics deteriorations by the thermal asperity defects be previously detected and sorted out. For the development of products, too, there is a growing demand toward a method of estimating the likelihood of characteristics deteriorations thereby achieving a structure less likely to offer the thermal asperity problem.
Never until now, however, is there any simple yet precise method that can imitate the deterioration mode of the aforesaid “local overheating plus vibration”.
Such being the case, the present invention has for its object the provision of a novel estimation method of estimating the deteriorations of a magneto-resistive effect device, by which:
(1) it is possible to imitate the deterioration mode phenomenon of the “local overheating plus vibration” in simple yet very approximate state, thereby detecting a device likely to undergo characteristics deteriorations due to the thermal asperity problem early at an initial fabrication stage, and
(2) it is possible to judge what specifications a device structure less likely to give rise to the thermal asperity defect is in at a products development stage.
It is here noted that the prior art that would seem pertinent to the invention of this application is JP(A)2000-312912. This prior art comes up with the temperature estimation of the electromagnetic characteristics of a magnetic head by heating it with laser light. The laser light is merely used for the purpose of increasing the temperature of the magnetic head in a non-contact state without recourse to a conventional contact heating mode. In other words, the method of JP(A)2000-312912 does never make use of laser irradiation for the purpose of imparting heat shocks to the device.